TRACE Tree Rings Archaeology Climatology Ecology

Tree rings and Geomorphological Processes
in a Mountainous Region (French Alps)
E. Weiss & T. Wils
Department of Physical Geography, Faculty of Geosciences, Utrecht University, P.O. Box 80115,
3508 TC Utrecht, The Netherlands; e-mail:
Introduction
Mass movements on slopes are regularly observed in mountainous regions. Assessment of
mass movement hazard risks requires, besides the spatial mapping of endangered zones,
insight in the temporal character of related processes. In many regions information about
these temporal aspects is not available. However, information about frequency, magnitude
and duration of mass movement processes is needed to reliably forecast possible future
changes.
Our research concentrated on tree ring- and tree stem characteristics in relation to the
periodicity of recent and formerly active landforms: landslides, hill slope debris flows and a
sub-recent active rockglacier.
Survey Area
The survey area lies in the Southern French Alps, about 100 km northwest of Nice (Fig.1.).
Fieldwork was done by students of the Department of Physical Geography in the central and
eastern parts of the Ubaye valley and surroundings. Altitudes vary from 1100 m to peaks
exceeding 3000 m. The tree line is found between 2000 m to 2400 m and is strongly affected
by man in many locations. In many cases the tree line is situated about 200 m below the
natural tree line. Lithology varies from very unstable thick series of black marls to hard
sandstones, limestones, dolomites and quartzite’s. Climate has a mountainous character
with Mediterranean influence. Precipitation peaks are found in spring and autumn, but also
heavy rainy summer storms can occur. The trees are relative young, mostly 100-150 years
old. On isolated places trees of more than 700 yrs can be found. At the highest elevation of
the tree line, Swiss stone pine and European Larch grow. Towards lower levels Mountain
pine and Scotch pine are found. Most study areas are positioned near the tree line at about
2000 m.
In the Rioux de Bourdoux drainage basin, northwest of Barcelonnette (Fig. 1), a complex of
active landslides in highly erodable marls overlain by moraine deposits was investigated on
periodic movement patterns. The drainage basin was strongly deforested by man at the end
of the 19 th century, but reforested up to 2200 m in the 20 th century. Common species are
Scotch pine in the lower parts and European Larch and Mountain pine on the higher parts. At
three landslide locations (A, B and C, Fig 3), between 1200 and 2000 m, tree ring eccentricity
was measured. For comparison location D was chosen on a relative stable slope without
moving parts (Braam et al. 1987a; Weiss, 1988).
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